Electron Self-Organizations and Transport Phenomena in High-Tc Cuprate Superconductors

人们认识到，由于强的电子关联，高T_c超导体中发生了某种类型的电子自组织。此外，在CuCuTe中，电子自组织现象似乎是其特殊电子态背后的原因，例如在强磁场下的异常绝缘正常态或超导带隙的本征不均匀。该项目通过使用高质量单晶的实验，特别是在极端条件下的输运性质的测量，阐明了铜酸盐中电子自组织现象的各个方面。主要结果如下：1.我们发现La_2-X_sr_xCuO_4中存在电荷有序的“魔术掺杂分数”，并用棋盘模型一致地解释了观察到的分数。对YBa2Cu3Oy、La2-xSrxCuO4和Bi2Sr2CaCuxO&lt；8+δ的超低温热导率的广泛测量揭示了d波bcs超导体普遍预期的“普适热导率”的分解。这种击穿显然与CuCuTe中特殊的超导带隙不均匀性有关。通过对存在自组织电阻率各向异性的非超导YBa2Cu3Oy输运性质的超低温测量，阐明了电子自组织态在非超导时具有本质绝缘性。此外，我们还发现这种绝缘状态具有特殊的对数(1/T)电阻率发散。我们证明了CuO_2平面内的电荷转移能可以从测量到1000K的霍尔系数的温度关系中得到。此外，我们还证明了霍尔系数数据可以用来解开超导掺杂下La_2-x_sr_xCuO_4的相分离性质。

It is recognized that some sort of electron self-organization occurs in high-Tc cuprate superconductors due to strong electron correlations. Also, it seems likely that in cuprates the electron self-organization phenomena lie behind their peculiar electronic states such as unusual insulating normal state under high magnetic fields or intrinsic inhomogeneity of the superconducting gap. This project has clarified various aspects of the electron self-organization phenomena in cuprates through experiments using high-quality single crystals, in particular their transport properties measurements under extreme conditions. Principal results are the following:1. We discovered the existence of "magic doping fractions" for charge ordering in La_2-X_Sr_xCuO_4, and showed that a checkerboard model consistently explains the observed fractions.2. Extensive measurements of ultra-low-temperature thermal conductivity of YBa_2Cu_3O_y, La_2-x_Sr-xCuO_4, and Bi_2Sr_2CaCu_xO_<8+δ> revealed a breakdown of the "universal thermal conductivity", which is generically expected for d-wave BCS superconductors. This breakdown is apparently related to the peculiar superconducting gap inhomogeneity in cuprates.3. Through the ultra-low-temperature measurements of transport properties of nonsuperconducting YBa_2Cu_3O_y, where a self-organized resistivity anisotropy is known to exist, we elucidated that the electron self-organized state is inherently insulating when it is nonsuperconducting. Moreover, we found that this insulating state is characterized by the peculiar log(1/T) resistivity divergence.4. We elucidated that the charge transfer energy in the CuO_2 plane can be obtained from the temperature dependence of the Hall coefficient measured up to 1000 K. Furthermore, we showed that the Hall coefficient data can be used for disentangling the phase-separated nature of La_2-x_Sr_xCuO_4 in the superconducting doping regime.